1. Field of the Invention
The present invention relates to an apparatus and a method for interleaving switching for a hybrid power modulator which modulates a supply voltage of a power amplifier according to an envelope of an input Radio Frequency (RF) signal in a radio transmitter.
2. Description of the Related Art
Wireless mobile communication terminals require power management integrated circuit development and wireless power amplifier efficiency increase for the sake of long battery lifetime. In a WirelssBroadband (WiBro) system and a Long Term Evolution (LTE) system, wireless mobile communication terminals require a technique for attaining high efficiency characteristics even with a high Peak-to-Average Power Ratio (PAPR). To address this issue, representative techniques such as Envelope Tracking (ET) or Envelope Elimination & Restoration (EER) may be used. These techniques change the supply voltage of a Radio Frequency (RF) power amplifier according to the output power and constantly operate the RF power amplifier in a saturated region or a switching region to simultaneously achieve high linearity and high efficiency. More particularly, the RF linear amplifier can attain high efficiency even with a modulation signal having a high PAPR.
FIG. 1 illustrates an envelope tracking power amplifier according to the related art.
Referring to FIG. 1, a modem 100 processes a baseband signal in conformity with a corresponding communication scheme (e.g., Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) communication scheme or Code Division Multiple Access (CDMA) communication scheme) and outputs the baseband signal to an RF module 102. The modem 100 also provides an envelope component of the baseband signal to a power modulator 106. The RF module 102 converts the baseband signal to an RF signal and outputs the RF signal to an RF power amplifier 104.
According to an envelope signal provided from the modem 100, the power modulator 106 modulates direct-current power (e.g., battery power) and outputs alternating-current power. The output signal of the power modulator 106 is used as a power source, thus achieving optimal linearity and efficiency.
The RF power amplifier 104 amplifies the RF signal according to the output signal of the power modulator 106 and outputs the amplified RF signal over an antenna.
FIG. 2 illustrates a hybrid power modulator for an envelope power amplifier according to the related art.
Referring to FIG. 2, a modem 200, an RF module 202, and an RF power amplifier 205 are substantially the same as the modem 100, the RF module 102, and the RF power amplifier 104 of FIG. 1, and thus their explanations shall be omitted here.
The RF power amplifier 205 uses a hybrid power modulator 204 including a linear amplifying unit and a switching amplifying unit. Most current flowing through the RF power amplifier 205 is supplied by the switching amplifying unit of the power modulator. When the output signal (hereinafter, referred to as a switching current) of the switching amplifying unit passes through an inductor 208, the linear amplifying unit pushes and pulls a compensation current in order to compensate for linearity distortion caused by ripple characteristics of the switching current. For example, the linear amplifying unit pushes the current to the output signal of the switching amplifying unit when the output signal of the switching amplifying unit is small, and pulls the current from the output signal of the switching amplifying unit when the output signal of the switching amplifying unit is great. A buck converter is generally used in the switching amplifying unit of the hybrid power modulator 204.
Typically, the linear amplifying unit includes a linear amplifier 214 and feedback resistors 210 and 212. The switching amplifying unit includes a comparator 216 for generating the switching signal, and a switching amplifier 218 for changing the switching signal of the comparator 216 to an ideal voltage or current level.
The switching signal in the hybrid power modulator 204 is compared with a reference signal by receiving the output signal fed back from the switching amplifying unit, to thus maintain the ideal output voltage. The comparator 216 for generating the switching signal, and the input signal and the reference signal of the comparator 216 determine characteristics of the switching signal. The frequency of the switching signal determines loss characteristics and ripple frequency characteristics of the buck converter. A comparator circuit of the related art includes a hysteretic comparator or a comparator which uses a Pulse Width Modulation (PWM) signal as the reference signal.
FIGS. 3A and 3B illustrate output signals and spectrums of a hysteretic comparator and a comparator that uses a PWM signal as a reference signal, which are used in a switching amplifying unit, according to the related art.
Referring to FIGS. 3A and 3B, the output switching signal 330 of a hysteretic comparator having one comparison phase of the reference signal maintains a certain output level when the input envelope signal 300 is greater than the reference signal 310, and produces the output level of zero when the input envelope signal 300 is smaller than the reference signal 310. The spectral properties 370 of the output switching signal 330 exhibit switching noise throughout a wider frequency band than the spectral properties 360 of the input envelope signal.
By contrast, the switching signal 350 generated using the PWM signal 340 as the reference signal is similar to the spectral properties of the input envelope signal and generates ripple noise power 382 intensively in a cycle frequency of the PWM signal which is used as the reference signal. The cycle frequency of the PWM signal 340 is mostly set to more than two times the input signal band. As the cycle frequency rises, the ripple frequency increases and the ripple noise level reduces. However, the switching loss increases to deteriorate the efficiency of the buck converter.
The integrated circuit for the hybrid power modulator of the wireless terminal is generally designed using Complementary Metal-Oxide Semiconductor (CMOS) process. Typically, a transistor used as the switch of the buck converter includes one power cell including one P-type Metal-Oxide-Semiconductor (MOS) Field Effect Transistor (FET) and one N-type MOS FET. Mostly, the buck converter uses a multiphase switching signal which reduces the ripple noise or shifts the switching frequency into the high frequency without the switching loss.
FIG. 4 illustrates a multiphase switching scheme according to the related art.
Referring to FIG. 4, the buck converter using the multiphase switching signal requires power cells including P-type MOS FET and N-type MOS FETs, and external inductors as many as the multiphase signals. For example, to generate N-ary phase signals, the buck converter needs N-ary MOS FETs, N-ary N-type MOS FETs, and N-ary inductors.
FIG. 5 illustrates a ripple noise reduction of a multiphase switching signal according to the related art.
Referring to FIG. 5, in the buck converter using the multiphase switching signal, the ripple noise frequency exhibits in the N-time frequency, the ripple noise frequency 4fCLK exhibits in the 4-time frequency by adding the ripple noise frequency 500 in the N-time frequency when the phase is zero degree, the ripple noise frequency 510 in the N-time frequency when the phase is 90 degrees, the ripple noise frequency 520 in the N-time frequency when the phase is 180 degrees, and the ripple noise frequency 530 in the N-time frequency when the phase is 270 degrees, and other ripple noise frequencies 1fCLK, 2fCLK, and 3fCLK are reduced.
A DC-DC conversion circuit implemented using the integrated circuit through the CMOS process includes the P-type MOS FET and the N-type MOS FET for fulfilling DC-DC conversion through switching regulation of the battery current. In the DC-DC conversion, the loss in the P-type MOS FET and the N-type MOS FET includes largely two types. One is switching loss due to the switching frequency and the parasitic capacitance, and the other is conduction loss due to on-resistance of the transistor of the P-type MOS FET and the N-type MOS FET. As the switching frequency rises and the parasitic capacitance increases, the switching loss increases.
The buck converter used in the hybrid power modulator of the related art can increase the frequency of the ripple signal by raising the frequency of the switching signal and shift the ripple noise into the intended band. In a system demanding a high data transfer rate, a high switching frequency is required to process the signal in the wide frequency band. However, the high switching frequency deteriorates the efficiency of the buck converter because of the switching loss. In addition, on account of low resistance of the CMOS substrate, the output pulse signal of the buck converter is leaked to cause considerable noise. The considerable noise caused by the buck converter directly affects a Radio Frequency Integrated Circuit (RFIC) and a Power Amplifier Module (PAM) using the power management IC or the hybrid power modulator so as to increase the system noise. Hence, the switching scheme of the related art hardly satisfies both of the high efficiency and the high frequency.
The multiphase switching method of FIG. 4, which is one method of increasing the frequency of the ripple signal and reducing the ripple noise without additional switching loss, requires the power cells including the N-type MOS FETs and the P-type MOS FETs, and the inductors as many as the multiple phases. The great number of the power cells increases the chip area, and the great number of the inductors increases the packaged chip size. As a result, the number of the external passive elements increases and the chip price rises. Naturally, the use of the multiphase switching scheme is restricted.
Due to the limited switching frequency use, it is difficult to design a high-efficiency hybrid power modulator for envelope modulation of a wide bandwidth. Since the noise component of the switching unit of the hybrid power modulator increases, the noise increases in the receiver band to thus degrade the reception of the receiver.
When the limited switching frequency is used, the linear amplifier should provide the push/pull currents for absorbing the ripple noise and provide the current dependent on the input signal in order to compensate for the signal of the wide bandwidth. Accordingly, the role of the linear amplifier having the relative low efficiency grows, and the great sink current provided from the output stage of the linear amplifier causes the power loss at the same time.
It is not easy to control the frequency of the switching signal according to the PWM or hysteretic scheme of the related art. Hence, in the system which changes the channel width based on the status of the multiband communication or the user, the fixed switching frequency or the unpredictable switching frequency can affect the receiver noise.
Therefore, a need exists for an apparatus and a method for reducing the efficiency decrease and the switching ripple noise due to the switching frequency increase, and for increasing the ripple frequency.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.